In the related art, a halogen lamp or a high intensity discharge (HID) lamp has been mainly used as a vehicular lamp, particularly, a light source of a headlamp, but recently, a vehicular lamp using a semiconductor light source such as a light emitting diode (LED) and a semiconductor laser (LD) is being developed instead of the halogen lamp or the high intensity discharge (HID) lamp.
Multiple light sources, which are controlled to be individually turned on and off, are mounted in the vehicular lamp. For example, in some instances, a light source for a low beam and a light source for a high beam are mounted in the vehicular lamp. FIGS. 1A and 1B are circuit diagrams of the vehicular lamp that is provided with the multiple light sources studied by the present inventors. In the drawings, a first light source 302 corresponds to the low beam, and a second light source 304 corresponds to the high beam.
A lighting circuit 400R of a vehicular lamp 300R in FIG. 1A is provided with a first drive circuit 410 and a second drive circuit 412 which correspond to the first light source 302 and the second light source 304, respectively. The respective drive circuits 410 and 412 are configured with (i) a converter for outputting constant current, or (ii) a combination of a converter for outputting constant voltage and a constant current circuit.
Power source voltage VLO is input to an LO terminal through a mechanical relay RY1. When the mechanical relay RY1 is turned on and the power source voltage VLO is supplied to the LO terminal, the first drive circuit 410 supplies drive current (lamp current) ILAMP1 to the first light source 302. Power source voltage VHI is input to an HI terminal via a mechanical relay RY2. When the mechanical relay RY2 is turned on and the power source voltage VHI is supplied to the HI terminal, the second drive circuit 412 supplies drive current ILAMP2 to the second light source 304.
In a vehicular lamp 300S in FIG. 1B, the two light sources 302 and 304 are connected in series. A common drive circuit 414 supplies common drive current ILAMP to a series connection circuit of the light sources 302 and 304. A bypass switch 430 is provided in parallel with the second light source 304, and a switch driver 432 turns off the bypass switch 430 when high-level voltage is input to the HI terminal. In this case, the drive current ILAMP is supplied to the second light source 304 such that the second light source 304 is turned on. When the HI terminal is at a low level, the switch driver 432 turns on the bypass switch 430. In this case, the drive current ILAMP is applied to the bypass switch 430 and the second light source 304 is turned off.
While the combination of the high beam and the low beam has been described here, the same problem may occur even in respect to a combination of other light sources. See, for example, Japanese Patent Application Laid-Open No. 2016-082691.